The present invention relates to a drive system for a compressor designed to deliver compressed air for the operation of a fuel cell system in a motor vehicle, wherein the drive system comprises an electric motor which in operation can be fed with electrical energy from the fuel cell system, furthermore the invention relates to an apparatus driven by electrical energy and to a method for the operation of a fuel cell system.
Fuel cell systems with such drive systems are intended, amongst other things, as power plants for motor vehicles, with the electrical energy delivered by the fuel cells being applied, after appropriate conditioning, to one or more drive motors which serve for the propulsion of a motor vehicle. A part of the output power of the fuel cell system is however also applied to the electric motor which is required in operation to drive the compressor.
In practice problems are associated with the starting up of the fuel cell system.
One known solution provides for the use of a traction battery with an operating voltage of, for example, 288 V. This traction battery has in principle three different tasks:
On the one hand it is used to drive the main compressor in order for this compressor to feed compressed air into the fuel cell system so that the power is generated which then replaces the traction battery as a power source for the electric motor driving the compressor.
The second task of a traction battery is to dynamically assist the electric motor or motors which propel the vehicle. For example, during heavy acceleration or at elevated speed the performance of the traction battery supplements the electrical output power of the fuel cell system.
The third task resides in the fact that the traction battery can be used to realize regenerative braking for example. That is to say, during the slowing down of the vehicle, the kinetic energy present is partly converted into electrical energy which can then be stored in the traction battery.
Although a traction battery can be useful for these different purposes it represents an expensive and heavy component so that one would like to dispense with the traction battery. If, however, the traction battery is dispensed with, it could no longer be used for the starting up of the fuel cell system.
In order to start up the fuel cell system air is required. However, the air compressor is driven by the fuel cell voltage and the latter is not yet available. In the absence of a traction battery it has already been proposed to supply the fuel cell system with sufficient air by means of a 12 V auxiliary blower, i.e. a so-called start-up blower, so that the power generation starts and the system gradually runs up until the power generation by the fuel cell system is sufficient to keep the system operating.
Irrespective of whether one operates with a traction battery or with an auxiliary blower, many components are required which complicate the system and make it more expensive, for example blowers, valves, piping, 288 V battery etc., and one would prefer to dispense with these.
The object of the present invention is to make sufficient air available for the fuel cell system to be started up and run up to power without the use of a traction battery or an auxiliary blower.
A further object of the present invention is to provide a general concept for the supply of apparatuses with electrical energy when starting operation of a fuel cell system, the concept being capable of being realized at a favorable price and of operating reliably.
Furthermore, the present invention is concerned with the provision of a new method for the operation of a fuel cell system or for starting up operation of a fuel cell system.
In order to satisfy these objects provision is made, in accordance with a first variant of the invention, for the starting of the fuel cell system, that the electric motor which drives the compressor can be driven with electrical energy from a low voltage battery and that the electric motor can be operated at an operating voltage which is significantly higher than the output voltage of the low voltage battery.
In other words it has been recognized, in accordance with the invention, that, despite the not inconsiderable difference in voltage between the output voltage of the low voltage battery and the operating voltage of the electric motor which drives the compressor, it is nevertheless possible to drive the electric motor (and thus also the compressor), at least at low speeds of rotation, to produce an adequate airflow in order to start up the fuel cell system, i.e. to place it in operation.
In this way it is possible to straightforwardly dispense with an expensive and heavy traction battery. A low voltage battery is in any case provided in the operation of a motor vehicle with a fuel cell system in order to supply the on-board electrics customary in the motor vehicle. However, this battery can be made smaller than in a motor vehicle with a customary power plant, since the need to supply a high starter motor current does not exist. This low voltage battery can now be used in a cost-favorable manner for the starting up of the fuel cell system.
In this connection it is particularly favorable when the compressor is a variable speed compressor and a restrictor device is provided at the air outlet side of a fuel cell system which has a restriction effect during the running operation of the fuel cell system but exerts no restriction effect, or only a comparatively small restrictor effect, for the starting up of the system.
It has namely been recognized, in accordance with the invention, that with a variable speed compressor a relatively high air throughput can be achieved when the back-pressure at the output side is kept small. Accordingly an adequate air-flow can be achieved even with relatively little power in order to set the fuel cell system operating.
In accordance with a preferred embodiment of the invention a switching device is arranged between the terminals carrying the operating voltage, i.e. the output terminals of the fuel cell system and the output terminals of the low voltage battery and selectively connects either the operating voltage or the low voltage to the electric motor or to a feed module connected in front of the electric motor.
The switching device should preferably be so designed that it produces a galvanic separation between the terminals carrying the operating voltage and the output terminals of a low voltage battery. In this manner it is possible to prevent the low voltage battery being loaded with the substantially higher operating voltage. Also a clear association is provided in this way between the respective power source and the electric motor. The switching device can for example be formed as a contacter, whereby the switching device can reliably perform many switching processes during the working life of the motor vehicle. Moreover the contactor represents a favorably priced solution for the use in accordance with the invention of a low voltage battery for the starting up of a fuel cell system.
A further embodiment of the invention lies in the provision of a voltage converter, which brings the output voltage of the low voltage battery to a voltage level which corresponds at least substantially to the operating voltage of the electric motor. For this purpose the voltage converter can for example be a lower power DC-DC converter which is sufficient to transform up the potential of the low voltage battery, of for example 12 V, to about 288 V. The required output power lies at around 200 W (in comparison the maximum output power of electric motor is about 7 kW), so that the corresponding current of the low voltage battery is under 20 Amps, from which it can be seen that both the DC-DC converter and also the low voltage battery can be designed in a cost-favorable manner for the purpose of realizing the invention.
It is particularly favorable when the electric motor is an AC motor with a feed module in the form of an inverter being connected in front of it, when the voltage converter is a DC-DC converter and when both the output of the DC-DC converter and also the operating voltage delivered from the fuel cells can be fed to the input of the inverter.
On the one hand the AC motor represents a favorably priced motor for the driving of the compressor and can be readily controlled from a power inverter (a so-called PIM power inverter module). On the other hand this solution offers the possibility of permanently connecting the output of the DC-DC converter to the terminals which carry the operating voltage, so that no special measures are required in order to switch the DC-DC converter on and off, since it only operates when the electrical system is switched on and the operating voltage is lower than the output voltage of the DC-DC converter. Thus no particularly expensive technical circuit measures are required in order to realize this embodiment.
It is however not essential to realize the electric motor as an AC motor; the use of a DC motor with appropriate control or with an appropriate feed module could also be considered.
Typical values for the output voltage of the fuel cell system, i.e. the operating voltage lie in the voltage range between 100 V and 500 V, whereas the output voltage of the low voltage battery that is used normally lies in the range between 12 V and 48 V and preferably amounts to either 12 V or 42 V.
As already mentioned the motor vehicle is preferably conceived to utilize the drive system of the invention in such a way that no traction battery is present. The compressor is, for example, a so-called screw compressor, since a compressor of this kind represents a main compressor which can be utilized for operation in the backpressure-free, low load case.
Other types of compressor can also be used which have a corresponding behavior.
In accordance with a further variant of the invention an apparatus driven by electrical energy is provided for the operation of a fuel cell system and can be fed in operation with electrical energy from the fuel cell system, the apparatus being characterized in that it can be driven by electrical energy from a low voltage battery for the starting of the fuel cell system, and in that it can be operated at an operating voltage which is significantly higher than the output voltage of the low voltage battery.
It has thus been recognized, in accordance with the invention, that the concept of driving apparatuses which are normally operated at high voltage using energy from a low voltage battery during the starting up of the fuel cell system can be extended to other apparatuses. For example, the concept can be extended to the driving of a hydrogen recirculation pump. The concept is, however, also capable of being used for the operation of other electrically operated components. Such other components are, for example, a heating apparatus in order to preheat a valve endangered by frost, or a heating apparatus which is used for generating water vapor.
In accordance with a third variant of the invention a method of operating a fuel cell system using at least one apparatus which is fed in operation with electrical energy from the fuel cell system is characterized in that the apparatus is driven for the starting of the fuel cell system with electrical energy from a low voltage battery, and in that, after the starting of the fuel cell system and the production of electrical energy by the fuel cell system, the apparatus is subsequently operated with high voltage energy from the fuel cell system.